The invention relates to a polyolefin film which comprises a polypropylene base layer and at least one top layer of a propylene polymer. Polypropylene films which comprise a polypropylene base layer and polypropylene top layers have been described in many publications. The films described are characterized by favorable properties, in particular, very good optical properties and scratch resistance. It is, however, impossible to process these films on automatic packaging machines, since they exhibit very poor heat-sealing properties. When using oriented films, the temperatures which must be applied for sealing are so high that severe shrinkage occurs in the sealing zone. The packages produced have a poor appearance and an inferior sealed-seam strength. In addition, the printing and coating properties of oriented polypropylene films need improving. A surface treatment of the films, such as corona treatment or flame treatment, may improve these properties, but this improvement is often inadequate or decreases rapidly during storage.
To obviate these disadvantages, oriented polypropylene films are usually provided with so-called sealing layers based on random copolymers of propylene and other .alpha.-olefins, polyethylene, or appropriate polymer blends. Such sealing layers have, however, the disadvantage that they impair the optical properties of the oriented polypropylene films. In opaque films gloss deteriorates, and in transparent films both gloss and haze are impaired. Scratch resistance must also be further improved. It is furthermore difficult to prepare copolymers of this kind, in particular if it is necessary to increase the amount of .alpha.-comonomers in order to improve the heat-sealing properties of the copolymer. The technical difficulties encountered in the production of heat-sealable raw materials on a basis of propylene copolymers are described, for example, in DE-A-29 23 754.
The coating of polypropylene films with dispersions containing polyvinylidene chloride or various polyacrylates also leads to sealable films, but these films are expensive and have the serious disadvantage that they cannot be reclaimed, i.e., film scrap which is inevitably produced during film production cannot be recycled due to the coating applied to it. For ecological reasons, this disadvantage is no longer acceptable.